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M-charts for failure frequency
We have a production process with very low (ppm-ish) fail rates that we would like to monitor for fails. Rather than monitoring ppm fail rates, it is a stronger test to monitor the number of fails (or time) between them, under the assumption that they follow a binomial distribution. In an M chart (this is what we call it) you then calculate a Z score every time there is a fail and chart as usual. It's not clear that JMP has this capability, but maybe it exists under a different name. Can anyone comment? We are using JMP 15.2 Pro.
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Re: M-charts for failure frequency
Click the Shewhart Variables button in the control panel on the left side of Control Chart Builder and select Rare Event. You can make a G-chart (negative binomial distribution model) or a T-chart (Weibull distribution model).
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Re: M-charts for failure frequency
Thanks Mark. Unfortunately that doesn't seem to quite be what I need. I can pre-process my data to convert from PPFPPPPPFPPF, etc. into counts of # of passes between each fail, and that has a negative binomial distribution. The strategy you recommend seems to calculate limits based on the percentile points of the observed distributions. However for a control chart I need to put in control limits based on a maximum allowed success (=Fail) probability (e.g., 1 ppm), rather than what is observed in the sample. Our general methodology is to map the binomial percentile to a z score using the inverse normal CDF, so that a value of -3 is a good lower limit; using that approach my data set averages around z = -4.7. The lower limit on the JMP chart is 0 however, which is essentially z = -INF. Do you have any recommendations?
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Re: M-charts for failure frequency
You can right-click on the chart and select Limits > Set Control Limits. You can now enter your pre-determined limits this way.